Page 77 - Carbonate Facies in Geologic History

64 Outline of Carbonate Petrography

basic microfacies have been illustrated by Horowitz and Potter (1971) disguised
by a few highly imaginative names such as "Satisfactory Succotash" and "Pleasant
Potpourri." Plates I to XVIII illustrate most of these sedimentary types.
The designated classes are attempts to interpret with the microscope what
Bathurst has aptly termed the "insignificant remnant of sea floor life and ecology"
furnished us by carbonate rock. Considering the range of sedimentological param-
eters which control deposition in the marine environment (depth, latitude, salin-
ity, water movement, light penetration, etc.) the organization of microfacies into a
limited number of categories is clearly an oversimplification. The suggested
grouping of them into standard facies belts of a generalized model as outlined
below (see Fig. II-5) contains some overlapping and inconsistencies and omits
many variations. But the grouping is applicable to enough known geological
facies complexes to demonstrate its general accuracy and to show the usefulness
of reduction to a limited number of types.
These types do not employ specific faunal and floral identifications but they
may be added as necessary when dealing with rocks of various geological ages. Of
course, in addition to sedimentological variations, biological changes through
geologic time strongly influence microfacies from System to System and compli-
cate the petrographer's task of interpretation.
In summary, despite obvious difficulties, a combination of the general paleo-
ecological observations of Fliigel with Dunham's or Folk's textural classes results
in very useful general categories. These are employed throughout the book, sym-
bolized in the general legend (Fig. III-1, SMF-1 to 24), are keyed into the facies-
environment schema of Fig. II-5 and in Fig. XII-3 in the final chapter.

Basin and Lower Slope Environments (Facies Belts 1 and 3)

SMF-l Spiculite (Plate II)
It is a dark, organic rich, and argillaceous lime mudstone or wackestone; siliceous
spiculitic ca1cisiltite. Spicules are usually oriented, generally siliceous monaxons,
commonly replaced by calcite.
SMF-2 Microbioclastic calcisiltite (Plate II)
This is a mixture of fine bioclasts and peloids with a very fine grainstone or pack-
stone texture. Fine ripple cross-lamination is common.
SMF-3 Pelagic lime mudstone (Plates III, XXIX)
Its micrite matrix contains scattered fine sand or silt grains composed of pelagic
microfossils (e.g., radiolarians or globigerinids) or megafauna such as graptolites or
thin-shelled bivalves like H alobia.

Slope Environments (Facies Belts 3 and 4)

SMF-4 Microbreccia or bioclastic-lithoclastic packstone
Grains are commonly worn and of originally robust character. They may consist of
both locally derived bioclasts and previously cemented lithoclasts; commonly they
are graded. Grains may be either polymictic in origin or of uniform composition.
Quartz and chert grains, as well as carbonate fragments, may be present. This rock
type includes both fine talus and coarser debris resulting from turbidites. The term
"allodapic limestone" of Meischner (1965) encompasses this microfacies.